An acid-base indicator which is a weak acid has a ${\mathrm{p}}^{\mathrm{H}}$ value = 5.5. At what concentration ratio of sodium acetate to acetic acid would the indicator show a colour, half-way between those of its acid and conjugate base forms ? ${\mathrm{pK}}_{\mathrm{a}}$ of acetic acid = 4.75

• A

  4.93 : 1

• B

  6.3 : 1

• C

  5.62 : 1

• D

  2.37 : 1